1. Field of the Invention
The present invention relates generally to electronic circuit device fabrication, and, more particularly, to the soldering of at least two different materials or members of such an electronic circuit device using a solder ball and alignment marks for aligning the elements to be connected, without using flux.
2. Description of the Related Art
When two different materials or members are to be soldered together, a flux or a cream containing flux has been used to remove oxide films formed on the soldered surfaces, to maintain these surfaces clean by preventing oxidization thereof and to promote the wettability of solder on these surfaces. However, when such a flux or flux-containing cream has been used, there has been a tendency for voids to be generated due to the evaporation of the flux during a soldering operation, as illustrated in FIG. 1(c), which results in a lowering of strength and hence reliability of the soldered junction.
Further, according to this conventional technique, environmental issues must be considered. More particularly, because the flux is typically washed by using an organic solvent such as a chloric solvent of Freon after the completion of the soldering operation, release of chlorofluorocarbons into the atmosphere results in the degradation of the atmospheric ozone layer. Restrictions on the use of such solvents further make the conventional technique unfeasible, even as it is very difficult to completely remove the flux by washing.
In an effort to solve this problem, a low-residual, low-activity flux requiring no post-soldering washing operation has been proposed. However, since such a low-activity flux is easily oxidized when heated in the atmosphere, the soldering operation must be performed in a belt furnace filled with N.sub.2. Such a furnace is partitioned by shutters into an inlet gas purge chamber, a heating, melting, and bonding chamber in which air is replaced by N.sub.2 sufficiently to maintain the oxygen concentration in this second chamber as low as about 70 ppm, and an outlet gas purge chamber.
Although this bonding method is easy to use, it requires the N.sub.2 -filled belt furnace. Therefore, fluxless soldering using an ion beam has been proposed in J. Vacuum Science Technology, 20(3), March 1982, pages 359-363. In soldering without flux, the oxide film must be prevented from growing on a solder surface after the surface has been cleaned by sputtering. To this end, the members to be soldered and the soldering material are aligned, and the soldering material is heated in a nonoxidizing atmosphere, which is, unfortunately, impractical in view of the facility needed to perform the operation. More particularly, a large, complicated facility is required, which inevitably results in technical difficulties in the alignment of the members.
Another fluxless soldering method has been proposed in Japanese Kokai (P) 58-3238, which describes the juxtaposition of two members to be bonded in a vacuum chamber, wherein the solder surfaces are cleaned by ion irradiation, aligned by overlapping, and then irradiated again with an ion beam to melt the solder. Since the cleaning, aligning, and heating operations are all performed within the vacuum chamber, workability and productivity are very low. More particularly, the method requires an alignment device that picks up at least one of the two members to be bonded, turns it over, carries it to the other member, and aligns a number of bonding portions on the two members. This operation requires a large-sized vacuum chamber, which increases the cost of the entire apparatus while decreasing its efficiency. Moreover, the possibility of contamination of the vacuum chamber is very high, and there is a limit to the thermal capacity of the apparatus due to the use of an ion beam, causing the simultaneous heating of large substrates to be difficult.
Another example is disclosed in Japanese Kokai (P) 3-171643. In this method, an atom or ion beam irradiation device and a post-processing device for aligning bonding portions and heating solder are separately provided. The interior of the post-processing device is filled with an inert gas. Members to be bonded together are aligned in a plenum chamber of the post-processing device and heated to a temperature below the melting point of the solder, under pressure, to temporarily fix the members, which are then transported to a heating/melting chamber of the post-processing device for the soldering to be performed.
Although no aligning device is required according to this method, the post-processing device must be kept at vacuum pressure, and the alignment and soldering performed in the post-processing device.
In spite of the problems of the prior art, the proliferation and advancement of computers and other apparatus that require electronic circuit devices employing flip-chip connections (wherein a number of fine connections are made using fine solder balls) requires a similar advancement in the precision of such connections. However, it has been very difficult to bond a number of parts using flip-chip connections within an inert atmosphere in a vacuum chamber with high precision. Furthermore, the transportation of such parts into, out of, and within the vacuum chamber, together with the evacuation of the vacuum chamber, etc., have proven troublesome, causing the workability of the prior art systems to be low.